The process of traveling-wire electroerosion generally makes use of a continuous wire electrode composed of, say, brass or copper, and having a thickness ranging between 0.05 and 0.5 mm. The term "wire" is used in the art not only to refer to a wire which is circular, triangular, square or polygonal in cross section but also include a like elongate electrode which may be in the form of a tape or ribbon. The wire electrode is axially transported continuously along a given continuous guide path from a supply to a takeup through a cutting zone in which a workpiece is disposed. The cutting zone is commonly defined by a pair of cutting guide members which support the traveling wire across the workpiece. Wire traction and braking means allow the continuous wire to be tightly stretched and kept taut between the supply and the takeup and to be axially driven between the cutting guide members while traversing the workpiece, thus presenting the continuously renewed electrode surface juxtaposed in an electroerosive cutting relationship with the workpiece across a narrow cutting gap. The cutting gap is flushed with a cutting liquid medium and electrically energized with a high-density electrical machining current which is passed between the wire electrode and the workpiece to electroerosively remove material from the latter.
The cutting process may be performed in any of various electroerosive machining modes. In electrical discharge machining (EDM), the cutting liquid medium is a dielectric liquid and the machining electric current is supplied in the form of a succession of electrical pulses. In electrochemical machining (ECM), the cutting medium is a liquid electrolyte and the machining current is a high-amperage continuous or pulsed current. In electrochemical-discharge machining (ECDM), the liquid medium has both electrolytic and dielectric properties and the machining current preferably is applied in the form of pulses which facilitate the production of electrical discharges through the liquid medium.
The workpiece may be immersed in a bath of the cutting liquid medium which serves to flush the cutting zone. Conveniently, however, the cutting zone is typically disposed in the air or ambient environment. One or two nozzles of the conventional design disposed at one or both sides of the workpiece have been utilized to deliver the cutting liquid medium to the cutting gap. The cutting liquid is conveniently water which is deionized or ionized to a varying extent to serve as a desired electroerosive cutting medium. It has been recognized that the role of the cutting liquid medium in the electroerosive process is to carry the erosive machining current, to carry away the cutting chips and other gap products, and to cool the traveling, thin wire electrode and the workpiece.
To advance the electroerosive material removal in the workpiece, the latter is displaced relative to the wire electrode transversely to the axis thereof. This allows the traveling wire electrode to advance translationally in the workpiece and consequently a narrow cutting slot to be progressively formed behind the advancing wire electrode, the slot having a width slightly greater than the diameter of the wire electrode. The continuous relative displacement along a precision-programmed path results in the formation of a desired contour corresponding thereto and subtly defined by this cutting slot in the workpiece.
Higher cutting speed is an ever increasing demand in the process described. It is, of course, desirable that higher cutting speed be obtained without loss of cutting accuracy. The cutting speed, typically expressed in mm.sup.2 /min, is defined by the product of the workpiece thickness and the length of cut achieved per unit time along a given cutting course and hence is, for a given workpiece thickness, dependent upon the rate of translational advance of the wire electrode that can be increased. If the rate of advance happens to exceed an actual rate of material removal which not only preset cutting parameters that govern, inter alia, the cutting accuracy but variable prevailing cutting conditions allow, the fine wire breaks so that the cutting operation must be suspended. The goal of higher cutting speed is, therefore, dependent on how ideally optimum conditions in the cutting gap may be established and with stability maintained against instantaneous changes. Among other factors which govern these conditions, it will be noted that adequate flushing is of particular importance.
It is desirable that the cutting gap defined between the traveling, thin wire electrode and the workpiece be kept flushed with a sufficient volume of the cutting liquid and traversed thereby at a sufficient rate to allow the electroerosive action to continue with stability, the cutting chips and other gap products to be carried away promptly and the wire electrode subject to erosive heating to be cooled with effectiveness. Thus, the art has seen various improvements in the structure of fluid-delivery nozzles and the manner of supply the liquid medium into the cutting zone. It has been observed, however, that they are no more than practical and far less than ideal. At best, some of them are only satisfactory to substantially increase the cutting speed when the workpiece is relatively thin. The greater the workpiece thickness, the more difficult it is to maintain the same cutting speed as attainable for thinner workpieces.